Intracerebral hemorrhage (ICH) is a debilitating disease that frequently afflicts premature infants. In this population, ICH most commonly originates in the germinal matrix, a collection of highly vascularized neural precursors located in proximity to the thalamus, and often extends into the ventricles. ICH results from compromise of the blood brain barrier (BBB) which in the mouse embryonic brain is composed of endothelial cells (ECs), basement membrane and mural cells known as pericytes (PCs). The overall goal of our project is to yield key insights into BBB formation and devise novel strategies to reduce the incidence and morbidity of ICH and intraventricular hemorrhage (IVH). Little is known about any signaling pathways in PCs that modulate the BBB and have implications for ICH. PDGFR- null mice lack brain PCs and have increased BBB permeability, but no brain hemorrhage or vessel rupture ensues. Transforming growth factor (TGF)1 protein levels are reduced in the germinal matrix compared to other regions of the midgestation human brain, and our preliminary results are the first to suggest that attenuating TGF signaling in PCs in the mouse embryo results in gross hemorrhage in the germinal matrix and thalamus and IVH. Embryos carrying PDGFR--Cre and floxed alleles for the TGF-receptor 1 (R1), have a relatively modest reduction in PC number but a markedly aberrant morphology and organization of ECs. In addition, it was recently suggested that neurovascular proteases are important players in germinal matrix hemorrhage, and PC-derived matrix metalloproteinase-9 is implicated as contributing to BBB breakdown in Alzheimer's disease. Our initial results in cultured cells and transgenic embryonic mice indicate that PCs with reduced TGF-R1 have increased expression of a disintegrin and metalloproteinase with thrombospondin motif (ADAMTS-1), a secreted matrix metalloproteinase that destabilizes capillary EC tubes. We hypothesize that mural cell TGF-R1-mediated signaling plays a key role in the development of an intact BBB and that disruption of this signaling pathway stimulates PC expression of factors (e.g., metalloproteinases) that induce aberrant EC polarization, migration and proliferation, thereby disrupting the BBB and causing ICH. To test this hypothesis, our studies will utilize transgenic mice, co-cultures of human brain PCs and ECs and human brain tissue. We aim to: i) elucidate the effects of TGF-R1-induced signaling in PCs on cellular processes in brain PC and EC development that are critical for BBB formation; and ii) determine the roles of metalloproteinases, and specifically ADAMTS1, in mediating the deleterious effects of PCs with reduced TGF-R1 on BBB formation and EC function. This R21 provides the foundation for a subsequent R01 application investigating in-depth the molecular mechanisms in PCs, ECs and/or surrounding tissues which underlie ICH. Taken together, our studies promise to yield seminal insights into BBB formation and ICH pathogenesis and suggest novel therapeutic strategies to prevent and treat ICH.